1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a signal path searching method and an apparatus thereof in a mobile communication system provided with a plurality of array antenna elements.
2. Discussion of the Related Art
Generally, if a smart antenna technique is applied to a base station, a gain through an array process is produced. Thus, if the power of a signal that a mobile station transmits is reduced by 1/N (where N is the number of antenna elements) or if an additional gain obtained by nulling an interference signal, the gain is reduced by 1/N or less where the base station uses a power control mechanism, and thus the power of the signals received through respective antenna elements of the base station is reduced in proportion to N.
FIG. 1 is a view illustrating the construction of a conventional base station receiver.
Referring to FIG. 1, a conventional receiver of a base station includes a scanning correlator 101 for searching a multi-path signals, a baseband processing section 102 for down-converting frequencies of respective path signals and converting the down-converted signals into digital signals, and a combiner 103 for combining the respective signals after compensating for a delay and a phase of the digital signals for each path.
The scanning correlator 101 obtains a partial correlation value between the digital signal and a Pseudo Noise code (e.g., a scrambling code) for searching each signal path at a specific time hypothesis. If the partial correlation value exceeds a specified threshold value, the scanning correlator recognizes that there is a new communication path at the specific time hypothesis. The receiver of the base station allocates a new rake finger to the new path. Each finger obtains an energy value of a symbol in each path on a temporal axis.
The combiner 103 maximal-ratio-combines the obtained symbol energy values obtained in all the paths, and a demodulator (not illustrated) demodulates the maximal-ratio-combined values.
The scanning correlator 101 serves as a searcher, is allocated for each antenna element, and searches successively identified signal paths.
For instance, in the path search algorithm of a scanning correlator of the conventional 3G asynchronous Wideband Code Division Multiple Access (WCDMA), the chip length of a scrambling code period used in one frame (assuming 15 slots) is of 38,400 chips, and thus a chip segment corresponding to one slot is of 2560. If it is assumed that a spreading factor of a certain control channel is of 256, the partial correlation value for a period of 256 chips is obtained at each time hypothesis for checking existence/nonexistence of a new signal path.
During the search process, the scanning correlator compares the partial correlation values with a specified threshold value. If the partial correlation value is larger than the threshold value, it decides that a new signal path exists.
Herein, the specified threshold value is set so as to satisfy a false alarm probability determined in the standard. If the threshold value is set to be high, the false alarm probability is lowered, but a detection capability becomes lowered.
On the contrary, if the threshold value is set to be low, the false alarm probability is heightened, but the detection capability becomes improved.
Meanwhile, in the 3G asynchronous WCDMA system, in order to match the synchronization of the scrambling code, the base station uses a Dedicated Physical Control Channel (DPCCH) of an reverse link dedicated physical data channel (DPDCH). The DPCCH, as shown in FIG. 2, is composed of pilot symbols for estimating channel information, a Transport Format Combination Indicator (TFCI) including Spreading Factor (SF) information of a reverse link channel, a FeedBack information (FBI) that is a feedback signal including information for a transmission diversity of a mobile station, and a Transmit Power Control (TPC) field including power control information. One frame of this DPCCH is composed of 15 slots.
However, according to the frame format, the number of slots transmitted for each frame may be changed, of which an example will now be explained.
Where a mobile station should perform an inter-frequency handoff, a base station should generate at least one empty slot that has no data bits. Since the at least one empty slot is generated with a different pattern for each frame, the number of the slots which each frame includes is changed.
Accordingly, during the path search by a receiver of the base station, a transmitted signal from the mobile station still includes its square component even if it is modulated. Thus, the transmitted signal is effective on the changed number of the slots. Accordingly, if it is known which slot is empty, the corresponding slot is skipped, and the slot where the data exists is searched.
This operation is applied to the respective elements of an array antenna in the same manner as the existing system.
A base station includes two kinds of searchers. One is a random access channel (RACH) path searcher, and the other is a traffic channel path searcher. Where a mobile station performs an initial connection to the base station through an up link channel, i.e., where the mobile station transmits an originating call or a destination call to the base station through a Random Access Channel (RACH), the searcher of the base station receives at least one preamble of the RACH and then transmits a confirmation signal for the preamble through a down link channel.
Accordingly, the mobile station transmits the signal subject to transmission to the base station.
During the above process, it is required that the base station accurately searches the preamble transmitted from the mobile station. For this, the RACH searcher of the base station successively performs a search operation. Herein, the searcher has a searching window of a specified size to search the RACH preamble.
When a call is established between the mobile station and the base station, the traffic channel path searcher successively performs a search in order to know whether a new signal path exists or not.
The traffic channel path searcher receives the time hypothesis when to be searched according to a control signal of a controller. The traffic channel path searcher obtains the partial correlation value at a specific time hypothesis, and compares the partial correlation value with a threshold value. If the correlation value is larger than the threshold value, the traffic channel path searcher recognizes that a new signal exists at the specific time hypothesis. Then the traffic channel path searcher allocates this new path to a finger of the rake receiver. A combiner combines the multi-path signals to maintain the performance of the receiver in an optimum state.
At this time, the base station obtains a combined gain by a method of combining the signals received from the respective antennas using two diversity receiving antennas. That is, a receiver of the base station includes the searchers that are independently operated for the two antennas. A modem of the base station finally combines the signals received through the antennas.
As described above, the base station including the conventional RACH searcher and the traffic channel searcher performs searches of the new signal paths using the separate searchers for the receiving antennas without using spatial information of the received signal. Therefore, the performance of the receiver of the base station is maintained by the maximum ratio combination according to the searched paths.
Where the searchers identical to those of the existing system are applied to the system that adopts the smart antenna, the Signal-to-Interference & Noise Ratio (SINR) of a combination signal of signals received through one antenna satisfies the signal quality to be requested on the link channel, but each SINR of the signals received through the respective antennas is reduced.
Accordingly, the method of receiving signals using one antenna cannot avoid the deterioration of the new path search performance. Consequently, the symbol error rate of the received signals is heightened due to the late search of the signal path for the optimum combination of the signals.
If the path search is performed by using the signal received through one antenna element, the deterioration of the new path search performance also becomes severe due to decrease of a power level of the received signal. Consequently, the deterioration causes the deterioration of the performance of the rake receiver due to a long search time.